Research is designed to yield new insights into basic mechanisms of heredity in of higher organisms. Projects investigate, from several avenues of approach, the nature cellular structures and processes involved in homologous chromosome pairing, genetic recombination, crossover interference, the maintenance of chiasmate association, and chromosome distribution to gametes. Techniques include cytogenetic engineering, laboratory experimental treatment and cytological observation. The organism selected for study offers a unique combination of excellence of meiotic cytology, an array of available meiotic mutants, morphological chromosome variants and polysmonics, a very good genetic map and probability of general applicability of results--Specific projects are focused on: (1) A search for specialized homologue pairing sites within chromosome arms based, on direct cytological observations of complex chromosome rearrangement heterozygotes; (2) Definition of the stage or stages of advance forms of homologue interaction in preparation for synapsis, utilizing newly developed and refined techniques which will all more precise observations than have been possible in the past; and (3) Tests of the nature of the mechanisms of chiasma maintenance and second division dyad integrity maintenance, by use of experimental treatment procedures, as well as observations of the behavior of rearranged chromosomes. --This research is expected to contribute to fundamental understanding of problems of great human concern. To the extent that integration of information of viral genomes into chromosomes and their rearrangement may play a role in oncogenesis, basic knowledge of the mechanism of chromosome pairing and recombination may be important to the study of the etiology of malignancy. The nature of the chiasmate association, and its induced disruption, has special relevance for understanding and preventing initial steps in the production of Down's syndrome and other aneuploid abnormalities. --Previous concern for deleterious effects of environmental contaminants has been focused largely on chromosome damage and mutagenesis. This work may provide impetus to serious consideration of hazards which may yield impairment of chromosome behavior at meiosis, and as a consequence, cause major birth defects.